Technical Lambda / Oxygen sensor diagnostics in a 1995 Barchetta

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Technical Lambda / Oxygen sensor diagnostics in a 1995 Barchetta

MichalM

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Hi Guys!

As I have been finally able to connect to my ECU I would like to ask your advice on my o2 sensor condition.

There has been a lot said on the forum about rough idle, but I have not seen any plots of actually what was happening to the fuel mixture when this occurred. I thought I would share these with you.

My B. is from 1995 so there is just one probe and it is located upstream the catalytic converter. This year has a throttle operated by a cable.

I've been experiencing rough idle which not only could be felt by chassis vibration but also could be hear standing by the exhaust outlet. Prior to checking o2 sensor I have tried to find any vacuum leak, but could not find any and cleaned MAF (although many advised against it I had some success in it - I did not break it while doing so ;) ). I have also excluded for now the ICV and replaced slightly worn gasket underneath it. Previous owner had the B. standing in the garage for quite some time and it might have contributed to the rough idle condition somehow. My fuel consumption is quite high, ca 12 l per 100 km at best. Then after watching couple of yt I started to suspect o2 sensor.

The test I have performed was following:
1) Checked lambda voltage and other related parameters at warm idle (not touching gas pedal at all)
2) Repeated the test at 2000 rpm
3) Checked wide open throttle condition twice to see how lambda responded to it

Between each step there was a couple of seconds that I did not record the data, but we can assume that I did the checks one after another in the order as above.

Here are the plots:

Idle check with timeline comments (started recording when engine was still a little cold)

11kcdhh.jpg


2000 RPM check

148gkew.jpg


Wide open throttle check (WOT)

jr8eox.jpg


Let me list some of my conclusions:
1) IDLE: Internet sources indicate o2 should be working in the closed loop while idling and cruising HOWEVER at idle check it did not go to closed loop operation which is abnormal
2) IDLE: There was a brief moment that lambda switched to closed loop just when there was some transition in the voltage reading
3) IDLE: Lambda voltage goes suspiciously low - less than 0.1V and around 0.05-0.08V, when voltage decreased below 0.2-0.3V the vibration started
4) 2000RPM: Lambda goes into closed loop operation normally and voltage sinewave pattern is visible
5) 2000RPM: There is no vibration or rpm oscillation while holding the gas pedal stable to keep the rpms steady
6) WOT: When I first pressed the gas pedal to the max position lambda switched to open loop as I think it should. During the second time (WOT2) lmabda switched back and forth.
7) WOT: During the second WOT the voltage does not show as rich indication as it did the first time.
8) WOT: Prior to this test lambda was at open loop but then after the test it was in the closed loop.

I am very interested in your input. At the moment I think about replacing the sensor. I think there is something wrong with it as the idle voltage is too low (maybe the heater doesn't work and it is too cold while idling) and it is in the open loop. Also, I think WOT response is somewhat poor.

What do you guys think?

Regards,
Michal
 
not being an expert at reading this graphics and after searching some information , i would risc to say you must have a lambda probe issue like intermitent working ( cracked element ) that is confusing the ECU manegement of the fuel misture , making it to allways ajusting from max. to min. causing the unstable idle ...
in the idle graphic the voltage is low telling the ECU the mixture is lean so it will inject more fuel than necessary to compensate , then some spikes !! i think the voltage value after some time would adjust and stablyse around an intermidiate value 0.5v ( not rich or lean)!! justifiing the heavy smell on exaust, high consumption of fuel and eratic idle !!
in full throttle the voltage values are similar from max. to min. they never stable but offcourse the vibration simptoms will not be notice...
this is my humble reading ! i hope i made it understandable and allways opened to corrections !
fuel issues like fuel pressure injector problems or MAF could also be a consern !
like allways you must go by eliminating a possible cause at time... the more dificult to eliminate are exactly the intermitent ones so i guess in this case you just be sure puting a new one and trying :eek:
 
Thanks JMSG, I will be changing the probe later this summer, will let you know if this was the issue. Basically, apart from vacuum leak that I could not eliminate and still suspect, I think I have eliminated everything apart from coolant temperature sensor and the probe. I have found a generic probe fitting B. from NTK at a reasonable price, the charges a on direct replacements from Denso are crazy in my humble opinion.
 
What software and connection device did you use to hook up to the car... I've tried a lot of different things the ODB2 doesn't give a lot of data - also are you connecting under hood or under dash ?
 
i don't have a clue about the price of a lambda probe !! have you consider just for trying a used one from scrap ? i think there are few types of them and they are very comon in diferent types of cars and brand !!
apreciate the feed back about result lather on ;)
 
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